Comparative wetting behavior of condensed and sessile drops on single micro-scale textured hydrophobic surfaces: Physical insights for condenser design

Abstract

Dropwise condensation (DWC) has gained significant attention in industrial applications for enhancing heat transfer efficiency. The wetting behavior of condensed droplets on patterned surfaces is crucial in DWC but has yet to be systematically investigated. In this study, we search for single micro-scale textured surfaces that can form Cassie condensed droplets. A total of 153 square-pillar-array-patterned hydrophobic substrates with various solid fractions and roughness factors were fabricated. An inverted optical microscopy system was applied to observe the wetting behavior of bottom-up growing condensed droplets and top-down depositing sessile drops on all substrates directly. Condensed droplets may exhibit different temporary wetting states simultaneously until their diameter exceeds 177.6 μm. Cassie condensed droplets can form through frequent dewetting transition on substrates with pillar-height ranging between 3.31 and 6.56 μm and pillar-spacing not exceeding 5.0 μm. Sessile droplets on superhydrophobic surfaces with excessively high pillar-heights always exhibit the Cassie state. However, condensed droplets on such surfaces exhibit an irregularly shaped Wenzel state instead, leading to filmwise condensation. Cassie and Wenzel DWC substrates demonstrate 28.3 % degradation and 36.9 % enhancement in experimentally obtained heat flux compared to a Teflon-coated flat surface. Our findings provide a guideline for the condenser's textured surface design to improve DWC.